Stressed-bond joint durability

The phosphoric acid-anodized process provides markedly improved stressed-bond joint durability and retards bond-line crevice corrosion (started at an edge) in severely corrosive environments when compared to chromic acid-anodized and FPL etched. (See Chapter 9 for description.)... 

Stressed-bond joint durability is markedly affected by the adherend prebond surface treatment and the adhesive/primer system in contact with it. This is evidenced by the poor performance of FM 123-L/BR 123 (non-CIAP) adhesive/primer system on FPL-etched and chromic acid-anodized 2024-T3 aluminum alloy, clad and bare, and the superior performance of the same systems when BR 127 (corrosion-inhibiting adhesive primer (CIAP)) is substituted for BR 123 (non-CIAP). 

The moisture content of both the wood and the adhesive affect the fracture behavior of adhesive bonded joints. Wood joints are especially sensitive to moisture effects as a result of the porosity and permeability of wood, which allows ready access by water to both the interior of the wood member and the adhesive layer. Irle and Bolton [57] showed that the superior durability of wood-based panels bonded with an alkaline PF adhesive compared to panels bonded with a UF adhesive was due to the ability of the phenolic adhesive to absorb and be plasticized by water. In the plasticized state, the phenolic adhesive is able to reduce stress concentrations that otherwise fracture the wood or the adhesive in urea-bonded panels. 

One of the earlier methods used by the Army for evaluating the durability of adhesive-bonded joints is the stressed temperature/humidity test described in ASTM D2919-01. Army ARDC workers at Picatiimy Arsenal... 

ASTM D 1828-70—Atmospheric exposure of adhesive-bonded joints and structures Determining durability of adhesive joints stressed in peel -Determining durability of adhesive joints stressed in shear by... 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

The durability of the bonded joints was greatly influenced by the nature of the adhesive the best performers in all climates were epoxy-novolak and nitrile-phenolic formulations. A tropical, hot-wet climate was the most damaging to bonded stmctures and the combination of high humidity and applied stress was particularly deleterious. During exposure to natural environments, the failure mode of aluminium joints was found to change gradually from wholly cohesive, within the adhesive, to include increasing amounts of interfacial failure (see Stress distribution mode of failure). 

Atmospheric Exposure of Adhesive-Bonded Joints and Structures. Practice for (D1828) Determining Durahility of Adhesive Joints Stressed in Peel, Practice for (D 2918) Determining Durability of Adhesive Joints Stressed in Shear by Ihnsion Loading, Practice for (D 2919)... 

The application of these principles to an actual adhesively bonded joint is anything but straightforward. One problem is the lack of pertinent information on the performance of adhesive joints. Most test data generated by adhesive producers are only useful for comparison purposes and are of limited use to the design engineer. Also, there is only limited information available on the performance of bonded joints exposed to service environments, while subjected to static or dynamic stresses. Adequate adhesive characterization and prediction of joint durability remain goals for the future. 

G. F. Carter, Outdoor durability of adhesive bonded joints under stress, Adhes. Age 10, 32 (1967). 

The interfacial regions have been highlighted as the regions where moisture may attack adhesive joints and the various mechanisms of environmental attack which have been postulated to explain the loss of durability are considered below. It should be emphasised that they should not necessarily be viewed as mutually exclusive mechanisms. Certainly there is ample evidence that no single mechanism can explain all the different examples which have been reported and which one is operative in any situation does depend upon the exact details of the adhesive system and the service environment. Also, it is noteworthy that the exact details of the mechanism of environmental attack may be dependent upon the timescale, temperature and stress level which the bonded Joint experiences, and these aspects are also discussed below. 

Despite the importance of designing out potential induced peel stresses in maximizing the strength of adhesively bonded joints, by far the most important factor is that the adhesive shear stress distribution is naturally highly non-uniform and that such joints would have extremely limited durability if this were not so. The variability comes about as the result of enforcing compatibility of deformations, as is explained in Fig. 6. 

The bonded joints between typical thin structural elements have an extensive capability to tolerate the load redistribution that is caused by local flaws and porosity with no loss whatever in strength or durability. This derives from the same minimum overlaps needed to provide resistance to creep rupture that were discussed earlier. Figs. 26-29, taken from ref. [19], address this issue in the context of the longitudinal skin splices in the PABST forward fuselage, where the thickness was 0.050 inch of 2024-T3 aluminum alloy. Fig. 27 shows the adhesive stress distribution at room temperature for a load of 1000 Ibs./in., which corresponds with a 1.3 x P proof pressure load. Significantly, this load does not even exceed the elastic capability of the adhesive for this environment. 

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